Cosmetic hair products aim to alter the appearance, feel and/or manageability of hair. For example, hair conditioners typically reduce hair fibre-fibre friction by use of cationic ingredients which are substantive to hair due to the isoelectric point of hair being around pH 3.67 (Robbins, C. R., Chemical and Physical Behavior of Human Hair, 4th Ed., Springer, 2002, p. 351). Hair styling agents also alter the manageability of hair, for example in Flohr et al. US2012/0183486A1 discloses a “composition for chemically modifying the internal region of a hair shaft” and states in § 54 that “an increase in weight occurs following treatment of hair by a composition of the present invention and that the physical properties of the hair, namely its stiffness, are increased, and moreover, these changes being resistant to wash out”. Hair colourants, perming agents and methods for straightening hair also typically alter the mechanical properties and surface of hair fibres in addition to their main functions—dyeing, curling and straightening the fibres, respectively. In the modern age, consumers demand high performance from the cosmetic hair products that they buy and the service they pay for in salon. Indeed, the desire for improved performance and combination effects e.g. dyeing and conditioning is continually increasing.
As a result of these changes in mechanical and surface properties of the hair fibres, methods and devices are used to measure these properties of hair. The measurements are useful for providing feedback to consumers as to the properties and healthiness of their hair fibres and also to guide research and development of new cosmetic hair products to provide improved performance. Krause et al. EP2295029A1, for example, discloses an “‘Omega Loop’ Measurement” in § 73 and 74 and states that the “amount of force necessary to compress the hair a defined distance is measured”. Other methods are mentioned in Robbins from pages 413 to 418 (Robbins, C. R., Chemical and Physical Behavior of Human Hair, 4th Ed., Springer, 2002, p. 351). Reference Vaynberg and Nall, J. Cosmet. Sci., 60, 135-141 (March/April 2009)—hereinafter referred to as “Vaynberg and Nall”—discloses a device that allows hair scientists to measure hair tress changes in stiffness and lubricity.
Nevertheless there is an unmet need for improved devices and methods for measuring the properties of fibres, particularly human hair fibres. Particularly, there is a desire for devices and methods that are more sensitive and can distinguish between small and incremental improvements in mechanical and surface properties of fibres. Indeed, the human hand is highly sensitive and can detect tiny changes in hair fibre friction and stiffness—thus small and incremental improvements in the performance of hair cosmetic products can result in a highly improved consumer experience.